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1d
revised What makes a context free grammar ambiguous?
added 244 characters in body
1d
comment What makes a context free grammar ambiguous?
@justin There is no fourth level in these trees.
1d
comment What makes a context free grammar ambiguous?
@justin You're welcome to add them.
1d
comment What makes a context free grammar ambiguous?
@justin You have to look at the derivation tree. Find a source explaining them, and draw the trees.
2d
awarded  Enlightened
2d
awarded  Nice Answer
Feb
9
comment Gaussian elimination algorithm performance
They always mention the block Lanczos algorithm in this context.
Jan
25
awarded  Enlightened
Jan
25
awarded  Nice Answer
Jan
23
comment Prove that if $\mathcal L$ is context-free so $\mathcal L_{\text{even}}$ is context-free
@NehoraiRaphael Your expression does define the set of even-length strings, which is however different (in general) from $\mathcal{L}_\mathrm{even}$, which is the set of even-length strings in $\mathcal{L}$.
Jan
23
comment Prove that if $\mathcal L$ is context-free so $\mathcal L_{\text{even}}$ is context-free
@NehoraiRaphael Well, it doesn't work. I can explain exactly why, but I believe you can work it out yourself. I strongly suggest you spend a few hours reflecting on this matter on your own.
Jan
23
comment Prove that if $\mathcal L$ is context-free so $\mathcal L_{\text{even}}$ is context-free
@user1952009 There's only one way to know. Come up with the construction and prove that it works. Or check the literature for proofs that take this route.
Jan
23
comment Prove that if $\mathcal L$ is context-free so $\mathcal L_{\text{even}}$ is context-free
@NehoraiRaphael No, it's not. It's actually the prototypical non-regular language.
Jan
23
comment Prove that if $\mathcal L$ is context-free so $\mathcal L_{\text{even}}$ is context-free
@NehoraiRaphael No, it doesn't. Now spend a few hours on it before writing back. (No need to write back if all is understood!)
Jan
23
comment Prove that if $\mathcal L$ is context-free so $\mathcal L_{\text{even}}$ is context-free
@NehoraiRaphael Yes, $\mathcal{L}_\mathrm{odd} = \{a^nb^{n+1} : n \geq 0\}$ is not regular.
Jan
23
revised Finding the probability function of the maximal number
added 1 character in body; edited title
Jan
23
comment Prove that if $\mathcal L$ is context-free so $\mathcal L_{\text{even}}$ is context-free
@NehoraiRaphael $\mathcal{L}_\mathrm{odd}$ is not necessarily regular, indeed it is not regular in my example.
Jan
23
comment Prove that if $\mathcal L$ is context-free so $\mathcal L_{\text{even}}$ is context-free
@user1952009 This is a standard fact. One possible proof is through PDAs. Check your textbook.
Jan
23
revised Prove that if $\mathcal L$ is context-free so $\mathcal L_{\text{even}}$ is context-free
deleted 17 characters in body; edited title
Jan
23
answered Prove that if $\mathcal L$ is context-free so $\mathcal L_{\text{even}}$ is context-free